The process of localizing proteins in the outer membrane of Escherichia coli is extremely selective and efficient in that proteins indigenous to this location are rarely, if ever, found in another. We wish to understand the genetic basis for the observed selectivity and the molecular mechanisms involved in the transport of these non-cytoplasmic proteins to their respective cellular locations. We propose to study these problems using the technique of gene fusion. Techniques have been developed which allow us to isolate fusions between the lamB gene, which codes for the outer membrane protein, the lambda receptor, and the lacZ gene which codes for the cytoplasmic enzyme beta-galactosidase. The hybrid protein produced by these fusion strains is located either in the outer membrane or the cytoplasm depending upon the amount of gene lamB present in the hybrid gene. By isolating large numbers of lamB-lacZ fusions, we should be able to define the sequence in gene lamB responsible for localization. In addition, lamB-lacZ fusions which incorporate the hybrid protein into the outer membrane exhibit a characteristic phenotype which will allow us to select mutants defective in the export process. The characterization of the mutants should provide information regarding the mechanism of protein localization.